1. Field of the Invention
The present invention relates to GaAs and other III-V type semiconductor devices and methods of fabrication thereof. More particularly, the invention relates to the incorporation of a stabilizing layer on a III-V type substrate.
2. Description of the Prior Art
The prior art discloses the desirability of placing a coating on a semiconductor substrate in order to improve the thermal and electrical properties of the semiconductor device.
For silicon substrates, it is well-known that a metallization layer composed of one or more elements can provide either a Schottky barrier or ohmic contacts, depending on the components used for the metallization layer. Many of these metallization layers have the requisite amount of thermal and electrical stability needed for useful commercial applications when used with a silicon substrate.
Several types of coatings, both single metal and alloys, have been proposed for use with a substrate composed of III-V type elements, like gallium arsenide (GaAs). However, many of the prior art metallization layers which had been used with silicon substrates, have been shown to be unsuited for use with a substrate composed of a compound of Type III and Type V elements, like Gallium-Arsenide (GaAs), by virtue of being chemically unstable when abutting the III-V type substrate.
In such cases, a chemical instability causes the metallization coating to preferentially react either with the Ga or As. This reaction causes large regions containing excess Ga or excess As to form within the junction. In the worst case, the Ga component migrates through the entire coating to the contact surface where gallium oxide can form. The gallium oxide causes nonuniformities in the device and results in unstable electrical properties. The degradation in the GaAs device is increased over time and as the device is stressed.
Metal coatings predominantly comprised of a single metal abutting GaAs which have been examined include the following, each showing the formation of a Ga-rich phase and migration of that Ga-rich phase through the entire junction: (a) Pt on GaAs ["Reaction of Sputtered Pt Films on GaAs" by V. Kumar, J. Phys. Chem. Solids 36, 535-541 (1975); "The Effects of Surface Treatments on the Pt/n-GaAs Schottky Interface" by A. Aydinli and R. J. Mattauch, Solid-State Electron. 25(7), 551-558 (1982)]; (b) Pd and Ge-Pd on GaAs ["Metallurgical and Electrical Characterization of Metal-Semiconductor Contacts" by G. Y. Robinson, Thin Solid Films 72, 129-141 (1980); "Interaction of Evaporated Palladium Thin Films with GaAs" by A. Oustry, M. Caumont, A. Escaut, A. Martinez, and B. Toprasertpong, Thin Solid Films 79, 251-256 (1981)]; (c) the In and In-Pt on GaAs systems ["The Effect of the Metallurgical Properties of the In-GaAs Interface on the Electrical Properties of Ohmic Contacts to GaAs" by A. K. Kulkarni and T. J. Blankinship, Thin Solid Films 96, 285-290 (1982); "In/Pt Ohmic Contacts to GaAs" by D. C. Marvin, N. A. Ives, and M. S. Leung, J. Appl. Phys. 58(7), 2659-2661 (1985)]; and (d) Au on GaAs ["Disassociation of GaAs and Ga.sub.0.7 Al.sub.0.3 As during Alloying of Gold Contact Films" by E. Kinsbron, P. K. Gallagher, and A. T. English, Solid-State Electron. 22, 51-524 (1979); "Electron Microscope Study of Alloying Behavior of Au on GaAs" by K. Kumar, Jpn. J. Appl. Phys. 18, 713-716 (1979)].
Alloys which have been studied include the following, each showing the formation of a Ga-rich phase and migration of that Ga-rich phase through the entire junction: (a) the extensively studied Au-Ge and Au/Ni-Ge on GaAs contact systems ["Metallurgical and Electrical Properties of Alloyed Ni/Au-Ge Films on n-Type GaAs" by G. Y. Robinson, Solid-State Electron. 18, 331-342 (1975); "Contact Degradation of GaAs Transferred Electron Devices" by C. J. Palmstrom, D. V. Morgan, M. J. Howles, Nucl. Instrum. and Meth. 150, 305-311 (1978); "Alloying Behavior of Ni/Au-Ge Films on GaAs" by M. Ogawa, J. Appl. Phys 51(11), 406-412 (1980); "Ohmic Contacts to n-GaAs Using Low Temperature Anneal" by J. G. Werthen and D. R. Scifres, J. Appl. Phys 52(2) 1127-1129 (1981); "Characteristics of AuGeNi Ohmic Contacts to GaAs" by M. Heiblum, M. I. Nathan, and C. A. Chang, Solid-State Electron. 25(3), 185-195 (1981); "Metallurgical Behavior of Ni/Au-Ge Ohmic Contacts of GaAs" by A. Iliadis and K. E. Singer, Solid-State Commun. 49(1), 99-101 (1984)]; (b) the Au/Ni-Sn on GaAs system ["Au/ Ni/ SnNi/ n-GaAs Interface: Ohmic contact Formation" by Aydinli and R. J. Mattauch, J. Electrochem. Soc. 128 (12), 2635-2638 (1981)]; and (c) the Au/In-Ge on GaAs system ["Metallurgical Reactions in Au/(In-Ge) Ohmic Contacts to GaAs" by C. R. M. Grovenor, Thin Solid Films 104, 409-418 (1980)].
While the above mentioned prior work teaches the desirability of producing a coating on a III-V type substrate which can be resistant to migration of either substrate constituent through the coating, that body of work does not teach how to select coating components to effect the migration resistance, so as to improve the thermal and electrical properties of the semiconductor device.
As is disclosed herein, the inventors have found that the inclusion of a rare-earth element as a component of the coating can effect a migration resistance for coatings abutting a III-V type substrate.
The inventors have found that a stable coating for a Type III-V substrate can be fabricated by using an alloy which includes both rare-earth and a non-rare-earth components selects so as to form high melting point intermetallics with both the anion and cation components of a III-V type substrate.
Moreover, dopants and surface wetting agents can be added to the compound to tailor electronic properties. In addition, a cover layer can be added to abut the top of the coatings so as to increase the resistance of the coating to either exposure or to better allow other layers to be fabricated on top of the coating.
Accordingly, it is an object of the present invention to provide a coating for use with GaAs or other III-V type semiconductor substrates, which results in an improved graded junction contact with the substrate, and which exhibits high thermal stability and satisfactory electrical properties.
It is also an object of this invention to create a graded junction contact to a III-V type substrate which can become resistant to through migration of Ga or As.
It is yet another object of this invention to provide a junction which becomes more stable over time.
A further object of this invention is to provide a coating for a GaAs which allows for the addition to dopants or surface wetting agents to the substantial coating components to further modify or control the electronic properties of the system.
It is another object of this invention to provide an effective coating for other types of III-V type substrates in addition to GaAs.